One hunted terrorists in the 1990s for the FBI and later helped build a case against 9/11 suspect Zacarias Moussaoui.

Another headed the federal task force investigating the Enron corporate scandal of the 2000s.

A third has argued more than 100 criminal cases before the Supreme Court as the Department of Justice’s deputy solicitor general.

They are among the team of high-powered lawyers Special Counsel Robert Mueller hired recently to examine Russian interference in last year’s U.S. presidential election, and to determine whether there was collusion between Moscow and Donald Trump’s campaign staff before he was elected president in November.

In total, Mueller has hired 13 lawyers since his appointment a month ago, and he plans to add several more, said his spokesman, Peter Carr.

More investigators to join team

The investigation, said Peter Zeidenberg, who served as an assistant special counsel in the 2005 investigation of former White House aide Scooter Libby, could eventually include “dozens of FBI agents and a significant number of prosecutors.”

The high-caliber hires have attracted attention for the lawyers’ depth of expertise. They underscore the stepped-up investigation by Mueller, who took over May 17, eight days after President Trump fired FBI Director James Comey.

Former FBI Director James Comey testifies during a Senate Intelligence Committee hearing on Capitol Hill, June 8, 2017, in Washington.

Trump has repeatedly dismissed the Russia investigation as a “witch hunt,” and declared “nobody has been able to show any proof” that he or his campaign team was involved in any improper activities with Russian officials.

On Thursday, the president lashed out at Mueller’s team, saying the probe is being “led by some very bad and conflicted people” — an apparent allusion to reports that several people hired by Mueller have made contributions in the past to Democratic candidates’ political campaigns.

Kushner, ex-Trump aides under scrutiny

As part of his investigation, Mueller is examining the finances and business dealings of Trump’s son-in-law and adviser, Jared Kushner, and several other associates, The Washington Post reported.

When Mueller was appointed special counsel last month, Deputy Attorney General Rod Rosenstein gave him a fairly broad mandate, with responsibility to investigate links and any coordination between the Trump campaign and the Russian government, and “any matters that arise or may arise directly from the investigation.”

“It’s not unlimited, but it’s pretty darn broad,” Zeidenberg, the former prosecutor, said of Mueller’s authority.

In probing the Trump associates’ finances, prosecutors will look to see whether any financial improprieties indicate a motive to collude with Russia, Zeidenberg said.

Follow the money to find collusion

“People say you’re looking for collusion, and not all this financial fraud,” Zeidenberg said. “Well, from a prosecutor’s standpoint, the financial fraud could very well be the motive for the collusion.”

Among his early hires were three partners, all former federal prosecutors, like Mueller, with deep litigation experience, poached from the law firm he left last month, WilmerHale: James Quarles, Jeannie Rhee and Aaron Zebley.

Before joining WilmerHale in 2014, a week after Mueller, Zebley spent more than 15 years as a federal prosecutor and FBI agent handling terrorism and national-security cases such as the 1998 bombing of the U.S. embassy in Tanzania and the Moussaoui case after the September 11 attacks in 2001.

Quarles joined the firm in 1975 after serving as an assistant prosecutor on the Watergate special prosecution task force. Rhee had been a deputy assistant attorney general before joining the private firm.

Highly experienced federal lawyers

Several others come from the ranks of the Department of Justice’s career lawyers.

Andrew Weissmann, a 26-year Department of Justice veteran, has held a variety of senior positions: chief of the department’s fraud section, head of the Enron Task Force from 2002-2005, and general counsel for the FBI when Mueller led the bureau.

Another much-noticed legal heavyweight on the team: Michael Dreeben, who, as deputy solicitor general for more than two decades, has litigated more than 100 cases before the Supreme Court and argued before all but one of the nation’s 13 regional courts of appeal.

Zeidenberg called Dreeben “one of the top legal appellate lawyers in the country.”

John Malcolm, another former federal prosecutor now with the Heritage Foundation, said Dreeben’s expertise could greatly benefit the Mueller team.

“Having Michael Dreeben on speed dial [close at hand], to be able to ask him questions about whether a particular set of facts could arguably fit within the parameters of a criminal statute would be a valuable tool to have,” he said.

Questions of independence

Mueller’s hires have not been without controversy. At least three lawyers working on his team, Quarles, Rhee and Sean Berkowitz, another former Enron Task Force director, have made generous campaign donations almost exclusively to Democratic candidates over the years, according to campaign records.

The disclosure has led Trump allies to question Mueller’s independence.

Former House Speaker Newt Gingrich tweeted Thursday: “Mueller is now clearly the tip of the deep state spear aimed at destroying or at a minimum undermining and crippling the Trump presidency.”

But outside Trump’s closest circle, Mueller, who has served under two Republican and two Democratic presidents, enjoys broad bipartisan support. And Rosenstein, responding to concerns Trump may order him to fire Mueller, told a Senate panel this week that he saw no reason to dismiss him.

Nick Akerman, a former Watergate assistant special prosecutor, said the partisan attacks on Mueller recall a smear campaign Republicans waged against Watergate Special Prosecutor Archibald Cox in the 1970s.

“Cox was a solicitor general for John F. Kennedy, he was a Democrat,” Akerman, now a partner at the Dorsey law firm in New York, said. “They tried to raise exactly the same issues with members of our prosecution team and staff.”